1. What is Allergic Disease Such as Bronchial Asthma
It is known that Type I (immediate) allergic reaction, which plays a central role in allergic diseases represented by bronchial asthma, allergic rhinitis, atopic dermatitis, is initiated by mutual action of an exogenous antigen such as pollen or house dust and immunoglobulin E (IgE) specific thereto. An allergen which has entered into the body is presented to helper T cells (Th cells) as an HLA ClassII molecule and a peptide fragment by antigen presenting cells such as macrophages, and Th cells are activated by antigen stimulation through T cell receptors and produce cytokines such as interleukin-4. Thereby production of a specific IgE antibody for the allergen by B cells is enhanced.
There exist receptors which bind to the produced IgE antibody with high affinity on the surface of the cells such as mast cell, basophil, monocyte and are referred to as high affinity IgE receptor (FcεRI). When IgE bound to FcεRI is crosslinked by polyvalent antigen, it is activated and various kinds of mediators are released. In other words, it is considered that the signal transfer from FcεRI into the cell triggers allergic disease such as bronchial asthma.
As mediators which mast cells release, there are preformed mediators such as histamine which is released to outside of the cells upon degranulation and mediators produced and released at an early stage of activation such as arachidonic acid metabolite. When these act on bronchi, bronchial smooth muscle contracts and airway becomes narrower due to swelling of mucosa, secretion of phlegm and so on, which causes asthmatic attack. When they act on skin, inflammation, swelling and itching occur and nettle-rashs and so on are caused. When they act on nasal mucosa, vascular permeability is increased and moisture in the blood gathers and a nasal mucosa swells up to cause a stuffy nose and bring about allergic rhinitis in which sneeze and a lot of nasal mucus are generated by nerve stimulation. When this reaction is caused in alimentary canal, enteric smooth muscle contracts, and enteric movement (peristaltic movement) abnormally increases to cause gastrointestinal allergy such as abdominal pain, vomiting and diarrhea.
As mediators released from mast cells, in addition to these, there are eosinophil chemotactic factor and cytokines which are accompanied with transcription and released after a delay through protein synthesis. This is considered as a cause of chronic inflammation (Non-Patent Document 1; Enshou-to-Men'eki (Inflammation and Immunity) vol. 7, no. 2, 1999, p. 165-171). Drawn by the eosinophil chemotactic factor and cytokine discharged from mast cells, eosinophil having strongly toxic chemical substance gathers to the site of allergic reaction and discharge chemical substances and cause a tissue injury. If this reaction is caused in bronchi, mucosa epithelium exfoliates, allowing an antigen to invade more easily, and allergic reaction is prolonged. As a result, asthma becomes refractory, in which airway hyperresponsiveness is enhanced, the airway becomes narrower due to swelling and phlegm and breathing cannot be performed freely, etc. The condition ranges from a symptom only with a chronic cough and phlegm to a serious condition with a fatal strong stroke. The number of patients has been increased steadily till now and is expected to increase further in the future, and development of an effective pharmaceutical drug is desired earnestly.
2. Existing Asthma Drug
At present, inhaled steroid as an anti-inflammatory drugs, β stimulant such as procaterol and xanthine derivatives such as aminophylline and theophylline as a bronchodilator are mainly used for the treatment of asthma. The inhaled steroid has a broad anti-inflammatory effect, and utility thereof as a therapeutic agent for asthma is high, but necessity of guidance of an appropriate inhalation method and existence of an asthmatic of steroid resistance have been pointed out (Non-Patent Document 2; ASTHMA 13-1, 69-73 (2000), Non-Patent Document 3; Naika (Internal medicine) 81, 485-490(1998)). The bronchodilator activates adenylate cyclase, an enzyme which produces intracellular adenosine 3′,5′-cyclic monophosphate (cAMP), or inhibits phosphodiesterase (PDE), an enzyme which decomposes cAMP, in airway smooth muscle and thereby increases the cAMP level in the cell and relieves contraction of the airway smooth muscle (Non-Patent Document 4; Naika (Internal medicine) 69, 207-214(1992)). It is known that increase in the intracellular cAMP level causes restraint of contraction in the airway smooth muscle (Non-Patent Document 5; Clin. Exp. Allergy, 22, 337-344(1992), Non-Patent Document 6; Drugs of the Future, 17, 799-807(1992)) and it is effective for improving asthmatic condition. It is known, however, that xanthine derivatives developes systemic side effect such as fall in blood pressure or cardiotonic action (Non-Patent Document 7; J. Cyclic Nucleotide and Protein Phosphorylation Res., 10, 551-564(1985), Non-Patent Document 8; J. Pharmacol. Exp. Ther., 257, 741-747(1991)), and that β stimulant is easy to cause desensitization while increased dose thereof produces side effects such as finger shivering and palpitation. Therefore, development of effective therapeutic agent for asthma free from such side effects is desired earnestly.
3. What Syk is
FcεRI has a basic structure common with the other immunoglobulin receptors (T cell receptor, B cell IgM receptor) and belongs to a superfamily referred to as multichain immune recognition receptor. FcεRI has a heterotetramer structure (αβγ2) consisting of respectively one α-chain and β-chain and two γ chains noncovalently bonded in the transmembrane region. The α-chain of FcεRI has two immunoglobulin (Ig) homologous domain in extracellular domain, and the immunoglobulin (Ig) homologous domain of C-terminal region binds IgE with high affinity (Non-Patent Document 9; E. J. Biol. Chem. 266, 1991, p. 2639-2646). The intracellular domain thereof is, however, relatively short, and even if the intracellular domain of C-terminal of the α-chain is cut off, there is caused no change in signal transduction. On the other hand, the extracellular domain of the γ-chain is short and it exists almost in the cell forming a homodimer with S—S linkage. Cut-off of the intracellular domain of the γ-chains brings about disruption of signal transfer, and the γ-chains are involved in intracellular signal transfer. The β-chain has a four-transmembrane structure, and both of the N-terminal and C-terminal ends exist in the cell. The β-chain has an effect of amplifying signal transfer, and intracellular signal transfer obviously attenuates when the β-chain is deleted. The β-chain and the γ-chain do not have endogenous enzymatic activity, and there is respectively a specific peptide sequence (immunoreceptor tyrosine-based activation motif: ITAM or antigen receptor activation motif: ARAM) domain based on two tyrosine residue in the intracellular domain. When subjected to tyrosine phosphorylation, they bind SH domain (Srk homology domein) of non-receptor type protein tyrosine kinase (protein tyrosine kinase: PTK) with high affinity.
As proteins tyrosine phosphorylated by aggregation of FcεRI, non-receptor type PTK such as Lyn, Syk and Btk, adapter molecule such as Shc and Grb2, PI3K, etc. have been identified in addition to the FcεRI β-chain and FcεRI γ-chain.
Syk is a molecule belonging to a subfamily referred to as Syk family along with ZAP-70 which is a PTK important in signaling through T cell receptor. It is not permanently associated with FcεRI γ-chain but it strongly bind ITAM of γ-chain tyrosine phosphorylated by Lyn after aggregation of FcεRI through SH2 domain of itself. It is known that Syk is subjected to autophosphorylation and phosphorylation by Lyn upon this binding and Syk causes further allosteric structural change, which enhances its activity (Non-Patent Document 10; J. Biol. Chem. Vol. 270. P. 10498-10502, 1995). The activated Syk induces formation of adapter molecular complex and activation of an enzyme and transfers a signal to the commom passway such as phospholipase CY (PLC γ), MAP kinase (MAPK) which is used by many receptors.
PLC γ is tyrosine phosphorylated by Syk and phosphatidylinositol-4,5-diphosphate (PI-4,5-P2) is hydrolyzed to diacylglycerol (DAG) and inositol-1,4,5-triphosphate (IP3). DAG induces activation of protein kinase C (PKC), and activation of PKC induces degranulation in combination with increase in the intracellular calcium level. In addition, Syk is associated with various adapter molecules having no kinase activity and having only SH2 domain and activates MAPK superfamily, and arachidonic acid metabolism is caused through phosphorylation of PLA2. Activation of ERK, p38, JNK, etc. is involved in cytokine production of mast cell through transcription factors such as AP1 (Non-Patent Document 11; J. Biol. Chem. Vol. 270, p. 16333-16338, 1995).
It is reported that tyrosine phosphorylation of intracellular proteins and phagocytosis reaction, which are caused by immunoglobulin G (IgG) receptor (FcγR) stimulation, are remarkably restrained in a macrophage derived from a Syk deficient mouse (Non-Patent Document 12; Crowley, M. T. et al., J. Exp. Med. 186:1027-1039(1997)). Therefore, Syk plays an extremely important role in phagocytosis by macrophage via FcγR, and its participation in tissue injury caused by antibody-dependent cellular cytotoxicity (ADCC) is shown. Furthermore, Syk is involved in B cell activation (for example, Non-Patent Document 13; J. Biol. Chem., 1992, Vol. 267, p. 8613-8619 and Non-Patent Document 14; EMBO J., 1994, Vol. 13, p. 1341-1349), GM-CSFIL-5 induced eosinophil survival (for example, Non-Patent Document 15; J. Exp. Med., 1996, Vol. 183, p. 1407-1414), activation of blood platelet caused by collagen stimulation (for example, Non-Patent Document 16; EMBO J., 1997, Vol. 16, p. 2333-2341).
Accordingly, Syk inhibitor is expected to be useful as a therapeutic drug for diseases such as diseases derived from immediate allergy reaction and delayed inflammatory reaction (for example, bronchial asthma, allergic rhinitis, contact dermatitis, urticaria, food allergy, conjunctivitis, etc.) and diseases in which antibody participates, eosinophilic inflammation, diseases in which platelet activation participates. Particularly, it is considered to be very useful if it acts in an Syk specific manner without inhibiting Zap-70 which belongs to the same family and is expressed only in T cells.
4. Existing Syk Inhibitor
(1) As a novel compound useful as a pharmaceutical drug having inhibitory activity on protein tyrosine kinase, particularly, Syk family tyrosine kinase, imidazo[1,2-c]pyrimidine derivatives represented by the following formula have been reported (Patent Document 1; Japanese Patent Laid-Open No. 2004-203748).
wherein R1 and R2 are hydrogen, lower alkyl, phenyl which may be substituted or heteroaryl, R3 is hydrogen, lower alkyl, cycloalkyl, phenyl which may be substituted, heteroaryl or aralkyl, and A is hydrogen, lower alkyl, cycloalkyl, R4, heteroaryl, OR5, SR5 or NR6R7.(2) Abignente E. et al. have disclosed imidazo[1,2-c]pyrimidine derivatives having anti-inflammatory effect represented by the following general formula:
wherein RA is carboxy, ethoxycarbonyl, carbamoyl or carboxymethyl; RB is methyl or methoxy; and RC is methoxy, and methyl or chloro. (for example, Non-Patent Document 17; IL Farmaco, 1991, Vol. 46, p. 1099-1110).(3) In addition, Yura T. et al. have disclosed imidazo[1,2-c]pyrimidine derivatives useful as an Syk inhibitor represented by the following general formula:
wherein RD is hydrogen, alkyl, carboxy, alkylcarbonyl or carbamoyl; RE is —XA—RG, heterocyclyl, carbocyclyl or a condensed ring; XA is S, O or NH; RG is aryl or heteroaryl; and RF is aryl or heteroaryl. (See for example, Patent Document 2; WO0183485).(4) As a compound having Syk inhibitory effect, there have been reported 2-anilino pyrimidine derivatives represented by the following formula:
wherein, Ar represents an aromatic ring group which may be substituted, and R2 represents H, halogen or a group represented by —X1—R2a respectively. (See for example, Patent Document 3; WO9818782).
In addition, there has been a report about Piceatannol which is a natural product derived from a plant (Non-Patent Document 18; J. Biol. Chem. 269: 29697-29703(1994)).
(5) As a compound having Syk inhibitory effect, a compound represented by the following formula has been also reported (See Patent Document 4; WO02096905A1). The compound shown herein, however, exhibited an inhibitory effect against plural protein kinases and had an inhibitory effect against GSK3 and Aurora2 at the same level as against Syk.
(6) Besides, a compound represented by the following formula has been also reported (Patent Document 5; WO2004016597A2).
(7) In addition, thiazole derivatives represented by the following formula have been also reported (Patent Document 6; WO2004087698A2).
(8) In addition, as a thiazole derivative, a compound represented by the following formula has been also reported (Patent Document 7; WO2004087699A2).

As stated above, plural Syk inhibitors have been reported till now, but these compounds had mainly pyrimidine skeleton and, in addition, showed an inhibitory effect against plural protein kinases and did not have a high Syk specificity.
Each of the compounds shown in above (1) to (8) inhibits not only Syk but also ZAP-70 expressing in T cells at the same level, and has poor selectivity.
5. With Regard to Known Aminopyridine Compounds
(1) A compound represented by the following formula has been also reported (Patent Document 8; WO2004041810A1). However, the compound shown herein showed inhibitory activity to plural protein kinases including Jak, and the selectivity for Syk was never in a satisfiable level.
(2) Besides, a diaminopyrimidine derivative represented by the following formula has been reported as a PKC-theta inhibitor (Patent Document 9; WO2004067516A1).
(3) In addition, 2-substituted-4-heteroaryl-pyrimidine derivatives as shown below are known as inhibitors of cyclin dependent kinase (CDK) (Patent Document 10; Japanese Patent Laid-Open No. 2003-528872).

In the formula, X1 is CH, X2 is S; or one of X1 and X2 is S, the other of X1 and X2 is N; Z is NH, NHCO, NHSO2, NHCH2, CH2, CH2CH2 or CH═CH; R1, R2 and R3 are independently H, alkyl, aryl, aralkyl, heterocycle, halogeno, NO2, CN, OH, alkoxy, aryloxy, NH2, NH—R′, N—(R′)(R″), NH—COR′, NH-aryl, N-(aryl)2, COOH, COO—R′, COO-aryl, CONH2, CONH—R′, CON—(R′)(R″), CONH-aryl, CON-(aryl)2, SO3H, SO2NH2, CF3, CO—R′ or CO-aryl, wherein the alkyl group, aryl group, aralkyl group, heterocyclic group and NH-aryl group can be substituted with one or more groups selected from halogeno, NO2, CN, OH, O-methyl, NH2, COOH, CONH2 and CF3; at least one of groups R1 and R2 is other than H when X1 or X2 is S; R4, R5, R6, R7 and R8 are independently from each other H, substituted or unsubstituted lower alkyl, halogeno, NO2, CN, OH, substituted or unsubstituted alkoxy, NH2, NH—R′, alkyl-aryl, alkyl-heteroaryl, NH(C═NH)NH2, N(R′)3+, N—(R′) (R″), COOH, COO—R′, CONH2, CONH—R′, CON—(R′)(R″), SO3H, SO2NH2, CF3 or (CH2)nO(CH2)mNR′R″, (CH2)nCO2(CH2)mOR′″, wherein n is 0, 1, 2 or 3, m is 1, 2 or 3; and R′, R″ and R′″ are each independently an alkyl group which can be the same or different.
[Patent Document 1] Japanese Patent Laid-Open No. 2004
[Patent Document 2] WO0183485
[Patent Document 3] WO9818782
[Patent Document 4] WO02096905A1
[Patent Document 5] WO2004016597A2
[Patent Document 6] WO2004087698A2
[Patent Document 7] WO2004087699A2
[Patent Document 8] WO2004041810A1
[Patent Document 9] WO2004067516A1
[Patent Document 10] Japanese Patent Laid-Open No. 2003-528872
[Non-Patent Document 1] Enshou-to-Men'eki (Inflammation and Immunity) vol. 7, no. 2, 1999, p. 165-171
[Non-Patent Document 2] ASTHMA 13-1, 69-73 (2000)
[Non-Patent Document 3] Naika (Internal medicine) 81, 485-490(1998)
[Non-Patent Document 4] Naika (Internal medicine) 69, 207-214(1992)
[Non-Patent Document 5] Clin. Exp. Allergy, 22, 337-344(1992)
[Non-Patent Document 6] Drugs of the Future, 17, 799-807(1992)
[Non-Patent Document 7] J. Cyclic Nucleotide and Protein Phosphorylation Res., 10, 551-564(1985)
[Non-Patent Document 8] J. Pharmacol. Exp. Ther., 257, 741-747(1991)
[Non-Patent Document 9] E. J. Biol. Chem. 266, p. 2639-2646,
[Non-Patent Document 10] J. Biol. Chem. Vol. 270. P. 10498-10502, 1995
[Non-Patent Document 11] J. Biol. Chem. Vol. 270, p. 16333-16338, 1995
[Non-Patent Document 12] Crowley, M. T. et al., J. Exp. Med. 186:1027-1039(1997)
[Non-Patent Document 13] J. Biol. Chem., 1992, Vol. 267, p. 8613-8619
[Non-Patent Document 14] EMBO J., 1994, Vol. 13, p. 1341-1349
[Non-Patent Document 15] J. Exp. Med., 1996, Vol. 183, p. 1407-1414
[Non-Patent Document 16] EMBO J., 1997, Vol. 16, p. 2333-2341
[Non-Patent Document 17] IL Farmaco, 1991, Vol. 46, p. 1099-1110
[Non-Patent Document 18] J. Biol. Chem. 269: 29697-29703 (1994)